I was just gifted a complete set of Rally Pac gauges from a friend for a 69 Cutlass Supreme. Which is awesome "BUT" I've replaced the factory wiring harness with a 21 circuit aftermarket harness. Anybody else done this kind of conversion?

 

So? The wires are still the same functions - oil, temp, and fuel level senders, brake warning light, gen warning light, illumination feed, and ground. Your aftermarket harness has to have these wires in it, just build the Rally Pac connector to match the pin locations as shown in your Chassis Service Manual.